Diuretics act at the apical surface of specific segments of the renal tubular epithelium.
Question: how do diuretics get to the apical surface?
One could imagine several such scenarios, such as diuretics getting filtered at the glomerulus and then traveling downstream through the urine to its specific site. This is not the case, however. It turns out that diuretics enter the blood stream, make their way into proximal tubule cells, and are actively secreted into the tubular lumen.
Thus, drugs which act on the mechanism of proximal tubule secretion--such as probenicid, used in the treatment of gout--will also inhibit diuretic activity, by blocking its trafficking.